Project Summary Hepatocyte nuclear factor 4 alpha (HNF4?) is considered the master regulator of hepatic differentiation because of its ability to regulate several hepatocyte specific genes. Recent studies from our group and others have revealed that HNF4? can inhibit hepatocyte proliferation and maintains adult hepatocyte quiescence. Our studies show that deletion of HNF4? in mature adult liver (HNF4?-KO mice) results in spontaneous hepatocyte proliferation with hepatomegaly without any tissue damage or injury. Further, HNF4?-KO mice demonstrate increased liver cancers. However, the mechanisms of HNF4?-induced inhibition of hepatocyte proliferation are not known. Preliminary data obtained using combinatorial RNAseq-ChIPseq analysis of HNF4?-KO livers indicate that HNF4? inhibits several promitogenic genes in quiescent hepatocytes both via direct histone deacetylation-mediated epigenetic regulation. Based on these data we hypothesize that HFN4? inhibits hepatocyte proliferation by inhibition of promitogenic gene expression via epigenetic mchanisms. Similarly, the role of HNF4? in liver regeneration, a process critical for liver homeostasis and which involves controlled proliferation of differentiated hepatocytes, is not known. Preliminary studies show that HFN4? expression and activity decline dramatically within first 12 hr after partial hepatectomy (PH) in mice and returns back to pre-PH levels at day 5 after PH. Decline in HNF4? activity coincides with initiation of cell cycle and return of HNF4? activity coincides with termination of liver regeneration. Further, we observed a termination/redifferetiation defect in HNF4?-KO mice after PH, which exhibited continued cell proliferation even at 7 days after PH. Based on these data we hypothesize that HNF4? reprograming is critical for both initiation and termination of liver regeneration. The studies proposed in this grant application will determine the previously unknown role of HNF4? in regulation of hepatocyte proliferation in normal and regenerating livers. These studies will demonstrate that HFN4? is a critical player in liver regeneration and could be a novel therapeutic target for regenerative therapies in various liver diseases including acute liver failure and liver cancer.